The present invention pertains to an apparatus for and a method of predistorting a complex baseband signal. More particularly, the present invention pertains to an apparatus for and a method of generating an amplitude modulated radio frequency signal by predistorting its baseband signal using the inverse hyperbolic tangent of a value based on the envelope of the baseband in-phase and quadrature signals.
The outputs from many solid state power amplifiers include distortion that can be characterized by a hyperbolic tangent function. Various radio applications, such as VHF data radio utilized by commercial airliners, must meet spectrum mask requirements imposed by regulatory agencies, such as the United States Federal Communications Commission. The transmit spectrum of such a radio signal can spread near the desired signal band if the envelope of the transmitted signal is not constant, particularly if the transmitter power amplifier is being driven into soft saturation. While spurious emissions might be reduced by predistorting of the radio frequency signal envelope just before transmission to the output power amplifier, this requires analog multipliers. Even then, if noise is picked up in the multiplier circuit, that noise will modulate the desired signal and pass through to the output.
The present invention is an apparatus for and a method of generating an amplitude modulation of a desired radio frequency signal such that after passing through the non-linear power amplifier, undesirable spurious emissions in the resulting spectrum are reduced. In accordance with the present invention, a complex amplitude modulated baseband signal, having an in-phase component I and a quadrature component Q, is sampled to obtain samples Ik of the in-phase component and samples Qk of quadrature component, and the magnitude of the envelope of the baseband sample is determined by finding the square roots of the sums of the squares of the in-phase component samples and the quadrature component samples. A distortion factor equal to the inverse hyperbolic tangent or archyperbolic tangent (xe2x80x9catanhxe2x80x9d) of a scaled value of the complex baseband sample magnitude divided by that scaled sample magnitude is used to multiply each sample of the in-phase component and of the quadrature component so as to provide predistorted components. These predistorted components are then upsampled, upconverted, and combined to provide a predistorted intermediate frequency (xe2x80x9cIFxe2x80x9d) carrier signal which is further upconverted to an analog radio frequency (xe2x80x9cRFxe2x80x9d) signal and filtered, leaving the desired upconverted signal.
If desired, the scaling factor can be obtained by combining a portion of the output signal envelope with the undistorted envelope in a feedback circuit. The feedback circuit preferably computes the mean square error between the undistorted envelope and the output signal envelope. Preferably, to assure that the mean square error is computed correctly, both envelopes are normalized. The mean square error is adjusted by a fixed gain control and integrated, and the result used to scale the undistorted envelope prior to determination of the archyperbolic tangent function.
The envelope of the baseband signal is thus subjected to digital envelope predistortion prior to upconversion. This avoids impressing pick-up noise on the transmitted envelope. It is possible to do the predistortion prior to IF and RF bandpass filtering of the radio frequency signal since such filtering has a wide bandwidth, allowing the distorted signal spectrum to pass through the power amplifier.
In the prior art to predistortion has been achieved by subjecting the signal to an inverse nonlinearity, for example, by passing the linear signal y(t) through an inverse nonlinear predistorter to generate f(y(t))=atanh(Cy(t)). In contrast, the present invention multiplies y(t) by {atanh(Cy(t))}/Cy(t), which can be considered an expander function.
Preferably, the predistortion apparatus of the present invention is implemented in a gate array, such as a field programmable gate array.